1. Field of the Invention
The present invention relates to electrochromic devices that have improved kinetics, improved counterelectrodes, and improved resistance to degradation from ultraviolet (UV) radiation. In particular, the present invention relates to electrochromic devices that incorporate i) reversible oxidizers to improve device kinetics, ii) counterelectrodes composed of an alkali metal oxide and vanadium oxide to improve stability, and iii) an electrochromic layer formed from doped tungsten or molybdenum oxide to improve UV durability.
2. Related Background
Electrochromic (EC) devices are used to make variable transmission and reflection glazing and mirrors that may be used for example in automotive and architectural applications. These EC devices can also be fabricated as light filters and displays for a variety of uses. Such devices can be used for energy efficient windows (architectural and transportation), skylights, automotive mirrors, displays, lighting control filters, etc. EC devices color and/or darken in response to an electric voltage. There are several types of electrochromic devices used to modulate light in a variety of applications. Most electrochromic devices have at least one electrochromic electrode which typically reversibly changes color upon ion insertion (reduction). For example, electrochromic devices may include i) an electrochromic layer (electrode) and ii) a redox material incorporated in an electrolyte layer. Many EC devices have another type of construction in which a liquid electrolyte or a polymeric solid electrolyte separate two electrodes, where at least one is electrochromic and where the other is a counter electrode (which could also be electrochromic) for ion insertion (see e.g., FIG. 1). The electrolyte may further include an anodic or cathodic dye. Many EC devices may also incorporate multiple thin coatings on one substrate.
Various examples of EC devices are found, for example, in U.S. Pat. No. 5,239,405 which describes electrochemichromic solutions, U.S. Pat. No. 4,902,108 which describes a single-compartment, self-erasing, solution- phase EC device, U.S. Pat. No. 5,729,379 which describes electrochemically active polymers, U.S. Pat. No. 5,780,160 which describes EC devices employing an electrochromically-inert reducing or oxidizing additive, U.S. Pat. No. 5,724,187 which describes EC devices containing redox reaction promoters, U.S. Pat. No. 4,671,619 which describes electrolytic solutions containing an iodide source material as a redox reaction promoter, and U.S. Pat. No. 5,725,809 which describes EC windows containing an ultraviolet stabilizer. Other examples are found in International Patent Publication WO 98/42796 which describes electrochromic polymeric solid films, International Patent Publication WO 97/38350 which describes an EC device containing a selective ion transport layer, and International Patent Publication WO 98/44384 which describes an EC device containing electroactive materials having a preselected perceived color. The disclosures of the above patents and publications are incorporated by reference herein.
The kinetics in EC devices, such as their bleaching and/or their coloration rates, disadvantageously decrease when the electrolyte thickness is increased because the electron carrier ions must travel longer distances. Therefore, it would be desirable to develop additives that, when added to the electrolyte, improve the coloration kinetics.
Further, it would be desirable to develop devices in which the coloration kinetics are insensitive to the gap (electrolyte thickness) between the electrodes. For example, in applications where curved glass is used over large areas (such as in automotive glass windows), large gaps of about 0.5 to 3 mm might be preferred between the glass plates. However, the glass bending tolerances can cause the interglass gap to vary from 10 to 50%. Such variations in the interpane gap distance can disadvantageously lead to non-uniform color due to the different coloration/bleaching rates for each gap distance. Devices having coloration kinetics that are insensitive to the gap would not exhibit such color variations.
A typical chromogenic layer utilized in EC devices such as a window is composed of, for example, tungsten oxide deposited on a transparent conductive substrate. A typical redox material used in the electrolyte is, for example, ferrocene. This electrolyte is sandwiched between the tungsten oxide layer described above and another transparent conductive substrate. When the EC cell is colored by applying an appropriate coloring potential, the tungsten oxide is reversibly reduced to a colored compound, tungsten bronze, while the ferrocene is reversibly oxidized to ferrocenium at the counterelectrode. When the bleaching potential is applied, (or under bleaching conditions) the ferrocenium oxidizes the tungsten bronze back to tungsten oxide, while the ferrocenium is itself reduced to ferrocene. The rate of such oxidation (or the bleaching rate) will depend in large part on the concentration of the ferrocenium near the bronze layer, the rate of transportation of ferrocenium through the electrolyte layer and the strength of ferrocenium as an oxidizer. Accordingly, it would be desirable to enhance the bleaching rate of EC devices in order to enhance their kinetics.
As described above, electrochromic devices can reversibly change light transmission or coloration when an electrical stimulus is applied. In many applications, electrochromic devices are subjected to not only visible and IR radiation but also UV radiation. Continued exposure to UV radiation can disadvantageously cause deterioration of materials and components, thereby leading to deterioration of the properties of the EC device. Thus, it would be desirable to minimize the change and/or degradation of these devices when subjected to UV radiation.
Many of the semiconductor materials utilized in EC devices can interact with other layers of the EC devices. For example, the semiconductor materials can undesirably interact with the electrolyte and transparent conductor layers when exposed to UV to shorten the EC devices' useful lifetimes. One of the effects from exposure to UV is the reduction in the transmission/reflection of the EC devices. This effect may be called "photochromism". Even more undesirably, the photochromism effect can be irreversible, i.e., cannot be reversed by applying a bleaching potential to the EC devices.
As described above, most electrochromic devices have at least one electrochromic electrode (an electrochromic or EC layer) that typically changes color reversibly upon ion insertion (reduction). Many EC devices use oxides of tungsten, molybdenum, or niobium as such electrochromic electrodes. The tungsten, molybdenum, or niobium oxides are often mixed with other oxides to change their color in at least one of colored and bleached states, spectral characteristics, ion-insertion/extraction properties, color/bleach rates, reversibility, durability, etc. It would be particularly desirable to eliminate photochromism in such EC devices which use at least one layer having a composition that includes tungsten oxide and/or molybdenum oxide and/or niobium oxide.
A typical prior art electrochromic device 17 is shown schematically in FIG. 1. EC device 17 can be used, for example, as a window. EC device 17 is in the form of a sequence of layers. A substrate 16 is adjacent to a transparent conductor layer 12' which abuts a counterelectrode (CE) 15. An electrolyte layer 14 is disposed between the counterelectrode and an electrochromic layer 13, which abuts a transparent conductor 12, which is adjacent to a substrate 11.
In a typical EC device, substrates 11 and 16 are often glass or a polymeric material, transparent conductor layers 12 and 12' are often formed from coatings of indium tin oxide or doped tin oxide, and electrochromic (EC) layer 13 is often an oxide (e.g., tungsten oxide, molybdenum oxide, etc.).
EC device 17 is typically constructed by taking two substrates 11 and 16 that are each already coated respectively with transparent conductors 12 and 12'. On the 11/12 coated substrate, EC coating 13 is deposited over the conductor coating. On the other coated substrate 16/12', CE coating 15 is deposited over the conductor coating. One of the coated substrates may be pre-reduced by ion-insertion. The two doubly coated substrates are then assembled together by, for example, lamination with an electrolyte or an ion-conductive layer therebetween. Examples of such devices are found in International Patent Publication WO 95/31746 which describes an electrochromic pane arrangement, International Patent Publication WO 97/22906 which describes an electrochromic element, and in U.S. Pat. No. 5,793,518 which describes an electrochromic system, the disclosures of each of which are incorporated by reference herein.
Another way in which EC devices with counterelectrodes are fabricated is by sequential deposition of thin films on one substrate. Schematically such devices resemble FIG. 1, but without second substrate 16. Examples of such devices are described in International Patent Publication WO 94/15247 which describes EC devices utilizing optical tuning layers, U.S. Pat. No. 4,712,879 which describes an EC mirror, S. P. Sapers, et al., "Monolithic Solid State Electrochromic Coatings for Window Applications", Proc. of the Soc. of Vacuum Coaters Conference, 1996, and U.S. Pat. No. 5,721,633 which describes a wholly solid type EC device, the disclosures of each of which are incorporated by reference herein. It would be desirable to produce novel coatings which can be used as improved counterelectrodes in EC devices employing at least one of tungsten, molybdenum oxide and niobium oxide, as EC coatings. Particularly, it would be desirable to produce novel counterelectrodes, for EC devices, with improved stability in the range of environments to which such devices are exposed. Some of these counterelectrodes reversibly change their color when they are reduced or oxidized in the devices.
The structure of the above described EC device is similar to a typical secondary (rechargeable) battery. In the above described devices, the EC layer is intercalated (charged or reduced) in the colored state of the device. That is, electrons are inserted into the EC layer from the counterelectrode in the coloration process or state. In the bleached state, the ions and the concomitant electrons are extracted from the EC layer and inserted into the counterelectrode. This process is again reversed for coloration.
Following the above analogy to secondary batteries, a counterelectrode commonly used for secondary batteries is vanadium oxide formed by processing the powder at high temperature (e.g., 500.degree. C. to 1500.degree. C.). The thus formed vanadium oxide(s) are then combined with other additives and binders before being applied as layered pastes onto metallic electrodes. Such vanadium oxide electrodes are typically opaque and dark in color due in part to the presence of sulfur compounds and carbon black powders. Although such optical properties are acceptable for batteries, typical EC device applications require that the electrodes possess optical clarity and optical uniformity--even after processing. Vanadium oxide (V.sub.2 O.sub.5) is used in certain EC devices as counterelectrodes although it is deep yellow in color. It would be desirable to change or vary the optical properties of such counterelectrodes.
Vanadium oxide can be modified by doping with other oxides, e.g., formation of vanadates. Many such materials are known, as described in A. F. Wells, Structural Inorganic Chemistry, 5.sup.th edition, Oxford University Press, Oxford, United Kingdom, 1991. Nevertheless, another disadvantage to the existing counterelectrodes utilizing vanadium oxide is the high processing temperatures required. To preserve the mechanical, chemical and functional integrity of the conductive substrates (e.g., soda-lime glass or plastic coated with transparent conductive coatings) used for EC devices, processing temperatures lower than 500.degree. C. are desirable. Accordingly, it would be desirable to produce compositions and processing methods for counterelectrodes that are useful in EC devices--including counterelectrodes containing modified vanadium oxides--that utilize lower processing temperatures.
A wet chemical method to add copper to tungsten oxide is described in U.S. Pat. No. 5,034,246. The patent describes adding a pyridine solution of Cu(II) acetylacetonate to a solution containing alkyl amine tungstate in order to produce a tungsten oxide film containing copper. The patent, however, does not discuss any benefits from such addition of copper. Nor does the patent discuss what the form of the copper was --whether the copper was in the final coating as an oxide or as elemental copper in a nano-composite.
Further, the patent does not provide any specific disclosure of the conditions necessary to perform the described copper addition.